Generally, the airspeed is estimated on board the aircraft from the measurement of two pressures:                firstly, the total pressure, obtained using a single Pitot probe; and        secondly, the static pressure, measured using static pressure probes which are flush with the skin of the fuselage of the aircraft.        
The Pitot probes and the static pressure probes are exposed to the outside conditions and can be disturbed by elements or objects which can partially or totally block the ducts upstream of the corresponding sensor, leading to incorrect pressure measurements.
It can therefore be useful to have an alternative method for estimating the airspeed of an aircraft which is independent of the Pitot probes. It is known practice in this context to use the lift equation, also called ascending force equation, which links four parameters, namely the vertical load factor, the incidence, the weight and the speed. The knowledge of the incidence, of the load factor and of the weight of the aircraft makes it possible to reconstruct an equivalent speed in real time. If, in addition, the static pressure is known, this also makes it possible to reconstruct the Mach number.
However, such an estimation of the airspeed, using the lift equation, is very accurate only in a limited part of the flight envelope of an aircraft for relatively low speeds. In particular, in the cruising flight envelope of an aircraft such as an airplane, the accuracy of the estimation of the airspeed is very degraded, even unusable.